Conventional semiconductor devices comprise a substrate and various structural layers thereon, in which individual circuit components are formed. The formation of various circuit components is partly accomplished by employing conventional photolithographic techniques to form a photoresist mask comprising a pattern and transferring the pattern to an underlying layer or substrate by etching the exposed underlying portions.
Memory devices, such as a NAND string, typically comprise a dual gate oxide structure. An exemplary dual gate oxide structure is illustrated in FIG. 1 and comprises select gate oxide 14 and tunnel oxide 12 formed on substrate 10. A conventional process for producing oxide layers having different thicknesses comprises initially growing an oxide layer over a designated region. A tunnel oxide mask is then formed on the oxide layer by depositing and patterning a photoresist material. The underlying oxide is then etched to remove the exposed oxide layer thereby exposing the silicon substrate thereunder. The photoresist mask is then stripped and a second oxide layer is grown over the entire region thereby forming a stepped oxide layer with two distinct regions having different thicknesses (12 and 14), as illustrated in FIG. 1.
The photoresist material is chosen to resist oxide etching and to protect the unexposed underlying oxide during etching, therefore, any residual photoresist in the exposed regions retards or even prevents etching of the exposed underlying oxide intended to be etched. Further, any residual photoresist in the exposed regions results in the formation of an uneven or a non-uniform surface of the second tunnel oxide film which may ultimately lead to device failure. Unfortunately, it is difficult to detect for the presence of unwanted photoresist in the exposed regions during fabrication.
Accordingly, in view of the criticality of the formation of the tunnel oxide layer, there is a continuing need for improved methods of detecting and removing unwanted photoresist from exposed regions of an underlying oxide layer prior to etching.